This invention relates generally to pressure regulators and deals more particularly with a low pressure pneumatic regulator which is especially useful in air distribution systems for the control of system powered terminal units.
Pneumatic control circuits of various types are often equipped with pressure regulators which provide a stable pressure signal that does not vary with the flow rate. In order to avoid adversely affecting the load circuit that is being controlled or powered, the pressure regulator should present a fixed low resistance to the load circuit. In many applications, it is also desirable for the pressure to be controlled thermostatically, such as when the regulator is used in air conditioning controls.
The type of pressure regulator which is most widely used includes a spring loaded diaphragm which closes against an exhaust orifice whenever the pressure on the exhaust side of the diaphragm is insufficient to overcome the spring force which opposes the exhaust side pressure. Usually, the force exerted by the spring is supplemented by the weight of the diaphragm and a backing plate which is typically attached to the diaphragm. A ball or other special device is sometimes provided instead of or in addition to the diaphragm to seal the exhaust opening. In nearly all cases, the regulated pressure is adjusted by varying the compression of the spring which urges the diaphragm toward the exhaust port. A screw or other threaded adjustment mechanism is normally provided to permit adjustment of the spring. Conventional residential gas valves have regulators of this type, as does the control device shown in U.S. Pat. No. 3,434,409 to Fragnito.
In order to adjust the spring force in a device of this type, considerable mechanical force and motion are required. Consequently, the use of a thermostat bimetal strip for setting of the pressure is impractical because it would be necessary to connect the bimetal element directly to the regulator and perhaps internally thereof. This would make the device difficult to calibrate and impractical to implement because of its physical bulk and thermal inertia. Also, the bimetal element could not be located remotely from the regulator as is usually desirable.
Thermostatically controlled regulators require operating power which is at least as high as the highest pressure that is to be transmitted to the load. If the main supply pressure is less than the maximum control pressure, the device cannot function as a regulator because the output pressure varies along with the supply pressure. In conventional pneumatic circuits, this does not present a significant problem because a compressor is provided which can supply pressures well in excess of the control pressures. However, in a system powered air conditioning control circuit of the type shown in U.S. Pat. No. 4,331,291 to Dean, the supply pressure is often rather low and can be below the pressure that might be desired as a control pressure at times when the system pressure is higher. For systems having these pressure conditions, it is desirable to use a thermostatically adjustable pressure regulator which functions effectively whenever the instantaneous supply pressure exceeds the control pressure needed at the time.
The control arrangement shown in the aforementioned Dean patent operates in an entirely satisfactory manner for the most part. However, when certain pressure conditions are present, problems can arise. For example, when the thermostat is partially closed, the state of each terminal unit is noticeably affected by variations in the duct pressure. If the duct pressure increases, the control circuit tends to close the terminal unit. Although good design of the duct system can compensate for this problem, poorly designed ducts are susceptible to disconcerting and inappropriate flow variations in individual terminal units due to system pressure fluctuations.